Influence of hydrothermal alteration on the elastic behaviour and failure of heat-treated andesite from Guadeloupe

Studies on the mechanical behaviour of rocks, including volcanic rocks, usually seek for unaltered and simple material: rocks without macroscopic defects. However, volcanic rocks are often naturally altered due to the circulation of hydrothermal fluids. This alteration may influence mechanical and physical properties. Here, we study the effect of hydrothermal alteration on the elasticity and failure properties of andesite. A homogeneous block of natural andesite was retrieved from a quarry. Three samples were first heat-treated and then artificially altered at different temperatures by soaking them in a brine for one month at a pressure of 20 MPa and temperatures of 80, 180 and 280 degrees C. Heat-treated unaltered and altered samples were hydrostatically loaded up to 50 MPa and unloaded, while strains and elastic wave velocities were recorded. Samples were also triaxially deformed to failure at a constant strain rate and a confining pressure of 15 MPa. At ambient pressure, increased alteration temperature resulted in increased wave propagation velocity, thus increased dynamic elastic moduli. During hydrostatic loading, volumetric deformation at a given effective pressure decreased with alteration temperature denoting increased static elastic moduli. During triaxial loading, the degree of alteration decreased elastic compaction and peak stress at failure. These observations are interpreted as the result of microcracks in-filling by alteration minerals, and in particular smectite, a swelling-clay mineral with a low friction coefficient. The mechanical behaviour of a volcanic rock subjected to triaxial loading was modelled with a damage model based on crack propagation from pre-existing flaws. A decreasing friction coefficient within the flanks of the cracks leads to a decrease of the peak stress and explains the experimental observations.